[CRIU] ***SPAM*** Re: [PATCH] Add inherit fd support

[Andrew Vagin]

On Mon, Dec 08, 2014 at 08:02:40PM -0800, Saied Kazemi wrote:
> There are cases where a process's file descriptor cannot be restored
> from the checkpoint images.  For example, a pipe file descriptor with
> one end in the checkpointed process and the other end in a separate
> process (that was not part of the checkpointed process tree) cannot be
> restored because after checkpoint the pipe will be broken.
> 
> There are also cases where the user wants to use a new file during
> restore instead of the original file at checkpoint time.  For example,
> the user wants to change the log file of a process from /path/to/oldlog
> to /path/to/newlog.
> 
> In these cases, criu's caller should set up a new file descriptor to be
> inherited by the restored process and specify the file descriptor with the
> --inherit-fd command line option.  The argument of --inherit-fd has the
> format fd[%d]:%s, where %d tells criu which of its own file descriptors
> to use for restoring the file identified by %s.
> 
> As a debugging aid, if the argument has the format debug[%d]:%s, it tells
> criu to write out the string after colon to the file descriptor %d.  This
> can be used, for example, as an easy way to leave a "restore marker"
> in the output stream of the process.
> 
> It's important to note that inherit fd support breaks applications
> that depend on the state of the file descriptor being inherited.  So,
> consider inherit fd only for specific use cases that you know for sure
> won't break the application.
> 
> For examples please visit http://criu.org/Category:HOWTO.

I don't have objection to commit this patch. Thank you for the work.

Acked-by: Saied Kazemi <saied at google.com>

Pls, look at an inline comment

> 
> Signed-off-by: Saied Kazemi <saied at google.com>
> ---
>  cr-restore.c         |   6 ++
>  crtools.c            |  15 ++++
>  files-reg.c          |  14 +++
>  files.c              | 241 ++++++++++++++++++++++++++++++++++++++++++++++++++-
>  include/cr_options.h |   1 +
>  include/files.h      |   7 ++
>  pipes.c              |  33 +++++++
>  util.c               |   5 ++
>  8 files changed, 321 insertions(+), 1 deletion(-)
> 
...
> +
> +/*
> + * Close all inherit fds.
> + */
> +int inherit_fd_fini()
> +{
> +	int ov;
> +	struct inherit_fd *inh;
> +
> +	list_for_each_entry(inh, &opts.inherit_fds, inh_list) {
> +		if (inh->inh_fd < 0) {
> +			pr_err("File %s in inherit fd list has invalid fd %d\n",
> +				inh->inh_id, inh->inh_fd);
> +			return -1;
> +		}
> +
> +		ov = inherit_fd_overwritten(inh);

I don't understand this moment. Could you show an example when a file
descriptor can be overwritten?

> +		if (ov < 0)
> +			return -1;
> +
> +		if (!ov) {
> +			pr_debug("Closing inherit fd %d -> %s\n", inh->inh_fd,
> +				inh->inh_id);
> +			if (close_safe(&inh->inh_fd) < 0)
> +				return -1;
> +		}
> +	}
> +	return 0;
> +}
...